Due to the maturity of wafer bonding technology and low MEMS (micro-electrical-mechanical systems) fabrication cost, borosilicate glass is widely used in current MEMS based sensors and actuators. Various types of pressure sensors which use this technology include differential pressure sensors, front side absolute pressure sensors, and backside absolute pressure sensors.
In situations where the glass has no major defects, such as those from the starting material from the fabrication process, the borosilicate glass used in the pressure sensors is strong enough to survive a high burst pressure, up to more than 100 bar for a typical structure shape and size. Glass defects may be, however, induced by the device fabrication process. It is the defects induced by the fabrication process that cause the glass to fail at a low pressure, for example, less than 50 bar, compared a structure having a similar size and shape having no defects. One of the major process steps which may induce glass defects is the wafer sawing to singulate the individual devices from the wafer stack, which often creates the mechanical defects around the sidewalls of the glass, such as cracking and chipping. If these defects are located in one of the high-stress areas, such as around the Si and glass interface, and around the die attach interface, the glass is more susceptible to failure when exposed to a high pressure. For example, in a differential or backside absolute pressure sensor, the glass may fail when around 30 bar (or less) of pressure.
The defect-free borosilicate glass wafers are normally polished on both surfaces, which may reduce the adhesion with a die mounting material, such as a paste, in a sensor device. The adhesion of the die with the mounting material may be degraded if the sensor is exposed to a hot and humid environment, or experiences thermal shocks, which often results in a device output instability and/or malfunction.
Accordingly, there exists a need for an MEMS device which is able to survive when exposed to a high burst pressure, and is robust in a harsh environment, which may include thermal shocks, and high humidity under an elevated temperature.